As a result of the increasing development of electronic driver-assistance systems and their integration into motor vehicles, the safety of the vehicle is being enhanced and the burden on the driver is being reduced. Examples are the anti-lock braking system, the electronic stability program and the spacing-regulated cruise control. Driver-assistance systems are able to intervene in the control of the vehicle with various degrees of automation: whereas with the lowest degree of automation merely prompts are indicated and the driver alone influences the motion of the vehicle, for instance by actuating the pedals or the steering wheel, with a higher degree the driver-assistance system intervenes supportively in devices of the vehicle. For example, the steering or the acceleration of the vehicle is influenced in the positive or negative direction. With the highest degree of automation a route is traversed substantially automatically, for example, or the locking of the wheels in the course of the braking procedure is prevented fully automatically.
With increasing degree of automation the driver-assistance system undertakes more and more automatic interventions. This is of particular significance when a critical situation is recognized and the system intervenes in the control of the vehicle. For instance, the spacing-regulated cruise control can reduce the speed of the vehicle to adapt the spacing to a vehicle cutting in. Furthermore, an emergency stop can be triggered, for example ahead of an obstacle suddenly appearing, or an automatic evasive maneuver can be carried out. The reaction triggered by a driver-assistance system may in these cases be effected distinctly more quickly than the driver himself/herself is able to grasp the situation and initiate countermeasures. In this connection, care has to be taken, in the interest of ride comfort and safety, to ensure that the driver is not surprised by the behavior of the vehicle and possibly reacts rashly. Therefore a warning is ordinarily output before or during the automatic intervention. The output of the warning message may, for example, be effected by a warning message in the instrument cluster of a motor vehicle, or by a warning tone.
In this connection it is to be noted that, depending on the driving situation, the attention of the driver may have been tied down in various ways, for instance, by unclear traffic events or by the manipulation of a device in the vehicle. The warning, therefore, has to be output in such a way that the driver registers it quickly, even if at this moment his/her attention is not directed directly towards the hazardous situation.